Generally, when it is intended to obtain high tensile strength in a continuous fiber nonwoven fabric, the tear strength tends to be reduced due to increase of bonding points, and these two effects cannot be easily attained at the same time.
Patent Document 1 describes a nonwoven fabric having excellent fuzz resistance as well as high tensile strength and high tear strength. This nonwoven fabric is a three-layer structure nonwoven fabric using a thermoplastic continuous fiber composed of a multiple component sheath-core fiber and consisting of a thermoplastic continuous fiber layer/a thermoplastic ultrafine fiber layer/a thermoplastic continuous fiber layer. A low-melting-point component on the outer side of the fiber having a sheath-core structure allows bonding of fibers to each other at a low temperature, and a high-melting-point component on the inner side does not participate in bonding, whereby high tensile strength and high tear strength are realized. However, this nonwoven fabric is liable to suffer from a problem that, for example, because of multiple component, the heat-resisting temperature is not high; or since a low-melting-point component is fused, a low-melting-point component must be contained and resinification of fibers is locally generated. For this reason, its application field is limited.
In Patent Document 2, it is proposed to use a thermoplastic continuous fiber for suppressing fuzz on the surface, and use of a composite thermoplastic continuous fiber composed of a plurality of components is proposed so as to obtain an adequate effect of suppressing fuzz and a sufficient tensile strength. However, in the case of using a multiple component fiber, as described above, a low-melting-point component is readily melted and this is liable to bring about, for example, a problem in the heat resistance or a problem of local generation of resinification. With respect to a nonwoven fabric using a thermoplastic continuous fiber composed of a single component, a method of laminating thermoplastic continuous fiber nonwoven fabrics together and using the laminate has been proposed, but in this method, interlayer separation readily takes place and high tensile strength cannot be easily developed, or although the separation may be suppressed by strongly press-contacting the fabrics, local resinification is liable to occur and it becomes difficult to obtain high tear strength. Calendering using an elastic roll is also disclosed, but when this technique is used, thermal pressure-contact in two stages is usually applied so that a heated roll can be contacted with each of the front and back surfaces, and there is a problem that in the case of a single component, the bonding effect by calendering in the second stage is insufficient and it is difficult to attain all of fuzz prevention, high tensile strength and high tear strength at the same time. This problem is presumed to be ascribable to the fact that crystallization of the nonwoven fabric has proceeded by the calendering in the first stage.
Patent Document 3 describes a nonwoven fabric with a three-layer structure of thermoplastic continuous fiber layer/thermoplastic ultrafine fiber layer/thermoplastic continuous fiber layer, which is composed of a single component prepared using a thermoplastic continuous fiber and has high tensile strength. It is disclosed that the ultrafine fiber of the interlayer intrudes into a gap between thermoplastic continuous fibers of the lower layer and thereby the nonwoven fabric exhibits excellent tensile strength as well as good filter and barrier properties, and that the tensile strength is more enhanced by applying flat calendering using a combination of a metal roll and a metal roll. However, the nonwoven fabric produced by the spun-bonding method has problems that dispersion of the basis weight is liable to be partially generated, impairing the uniformity of the entirety, the range of conditions to satisfy both fuzz prevention and high tear strength is narrow, and local resinification readily occurs.